Terrestrial Trunk Radio (TETRA) is a global standard for radio communications on private/professional mobile radios. Any TETRA standards or specifications referred to herein may be obtained by contacting ETSI at ETSI Secretariat, 650, Route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE. Professional mobile radios include radios, other than mobile telephones, such as mobile radios, portable radios, and the like. According to existing TETRA protocols, four channels are interleaved into one carrier using Time Division Multiple Access (TDMA) protocol, with a 25 kilohertz (kHz) carrier spacing. Each channel indicates a logical assignment on the carrier and is assigned one physical timeslot. Because four channels are interleaved into one carrier, a frame on the carrier is physically divided four timeslots, one for each channel. Hence, systems implementing existing TETRA protocols can support up to four simultaneous and independent calls on a single carrier.
In TETRA systems, channels are shared resources. A base station in a TETRA system automatically allocates the channels to radios at the beginning of each call. Radios thereafter transmit voice/data traffic or control/signaling streams on the assigned channels. Cost savings are therefore achieved in base stations where only one radio unit is needed for every four user channels. Both point-to-point (direct individual calls) and point-to-multipoint calls (group calls) may be made using TETRA protocols.
Existing TETRA protocols may be enhanced to further increase the carrier capacity. For example, existing TETRA protocols can be enhanced to support more simultaneous and independent calls on a carrier by further dividing physical resources. Existing TETRA protocols can also be enhanced to improve protocol services to support new multiplexing schemes. To support legacy TETRA units (base stations and mobile stations that are using existing TETRA protocols), enhancements to TETRA protocols need to be backward compatible so that legacy TETRA units can continue to work with units using enhanced TETRA protocols.
Accordingly, there is a need for a backward compatible method for enhancing TETRA protocols to increase the capacity of TETRA systems.